[GHC] #9939: Warn for duplicate superclass constraints

[GHC]

#9939: Warn for duplicate superclass constraints
-------------------------------------+-------------------------------------
        Reporter:  crockeea          |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by goldfire):

 Another example I hit recently. `Language.Haskell.TH.Syntax` used to have

 {{{
 class (Applicative m, Monad m) => Quasi m where ...
 }}}

 When I saw this recently, I realized the `Applicative m` constraint is now
 redundant and so I removed it. However, it would have been helpful if GHC
 had issued a warning.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9939#comment:1>
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